The European commission has demanded an "unqualified apology" from the BBC for what it described as the "disgraceful" treatment of one of its spokesman on Newsnight last month.

A letter to the director general, Mark Thompson, accuses Jeremy Paxman of losing control of the show after studio guest Peter Oborne described Amadeu Altafaj Tardio as "that idiot in Brussels". Tardio – who joined the programme by video link from the BBC's Brussels studio – walked out after Paxman admonished Oborne, a Daily Telegraph commentator, for being "gratuitously offensive".

But by then Oborne had used the phrase more than once in a debate about the eurozone crisis. The letter from commissioner Olli Rehn was copied to the BBC trust chairman and former EU commissioner Lord Patten. It says Paxman failed to make clear to Oborne after his first insult that such language was totally inappropriate.

It continues: "I was shocked by such an unacceptable behaviour as Peter Oborne's being effectively endorsed by a top figure of the BBC. When Mr Paxman finally reacted, after tolerating those insults several times, using the same 'idiot' language himself and having lost control of the programme, it was too late: the abused commission spokesman, who showed enormous patience and commitment, considered it was useless to try to have a rational discussion with interlocutors that resorted to insults instead of intelligent arguments, and took the right and difficult decision to leave. A shame, and improper practice for the BBC."